The present invention relates to the use of polyester fiber in textile fabrics, particularly blended fabrics, but also has application for the use of polyester as a container material for food and beverages.
As is well known to both those of ordinary skill in this art and to many end-users, staple fibers cut from polyester filament are often blended with natural fibers, including cotton fibers, to form blended yarns and fabrics that exhibit desirable properties of both the natural fiber and the synthetic polyester fiber. Blends of polyester and cotton are particularly suitable for such purposes and have found wide acceptance in consumer textile products, particularly clothing.
One of the problems associated with blended polyester and cotton fabrics, however, is the eventual appearance of "pills" on the blended fabric. A pill generally appears as a small ball of material on the surface of a fabric and for most textile products is considered visually and functionally unacceptable. Pills are formed of "a small accumulation of fibers on the surface of a fabric . . . and are usually composed of the same fibers from which the fabric is made." Dictionary of Fiber and Textile Technology, Hoechst Celanese Corporation (1990) page 113.
In blended fabrics, however, the pills tend to be almost exclusively formed from the polyester fibers in the blended polyester/cotton yarns. As best understood, the pills appear to be the result of the higher tensile strength of a typical polyester fiber as compared to the cotton fibers with which they are blended (typical cotton has about two-thirds of the tensile strength of typical polyester). The greater strength of the polyester fibers encourages them to work their way out of the yarn structure and then curl up upon themselves to form the pills. The tendency of a blended cotton/polyester fabric to "shed" cotton fibers also contributes to the problem.
It being understood that the greater strength of the polyester filament causes the pilling problem in blended fabrics, the basic attempts at solving the pilling problem have been to adjust the strength of the polyester filaments in some fashion in order to match them more closely to the strength of the cotton or other natural fibers with which they are blended.
In turn, it is generally well understood that the strength of a polyester fiber can be appropriately reduced by reducing the molecular weight of the polyester. To date, this has been accomplished by shortening the polymerization reaction times for the polyester. Reducing the reaction time and molecular weight, however, also tends to reduce the melt viscosity (and the related intrinsic viscosity, "I.V.") of the polymer, because under normal circumstances melt and intrinsic viscosity are directly proportional to the polymer average molecular weight. As known to those in this art, the melt viscosity is "the resistance of molten polymer to shear deformation," and is related to both intrinsic viscosity and temperature, Dictionary of Fiber and Textile Technology, Hoechst Celanese Corporation (1990) page 95. Intrinsic viscosity is the "ratio of the specific viscosity of a solution of known concentration to the concentration of a solute extrapolated to zero concentration, Dictionary of Fiber and Textile Technology, supra page 82.
Alternatively, branching agents such as pentaerythritol have been added in an attempt to reduce the strength of the polymer. In typical formulations, about 1000 ppm of pentaerythritol are added to produce a polyester with an I.V. of between about 0.45 and 0.5 dl/g. Although filament formed from this product exhibits lower tensile strength (and thus less pilling), its lower I.V. (and related lower melt viscosity) causes it to tend to run badly in a normal extrusion process. In particular, polymers in the melt phase immediately proceeding the step of spinning the melt polymer into filament, must have a certain intrinsic viscosity in order to undergo the various subsequent steps of filament manufacturer and finishing. If the melt phase has a relatively low melt viscosity (and low intrinsic viscosity), the filaments tend to break more easily during the manufacturing process. In turn, filament breakage causes severe productivity problems in the overall manufacturing scheme. Stated differently, the equipment used to produce polyester filament is designed to handle polyester with a certain I.V.; typically about 0.55 dl/g or greater. Attempting to use a lower I.V. material in the same machinery tends to result in a high level of process interruptions, typically spinning breaks and difficulty in pack wiping.
As noted above, attempts to solve the pilling problem have included adding other materials to the polyester to increase its viscosity at lower molecular weights. Such additives, however, often have cross-linking characteristics that tend to reduce the thermoplastic characteristics of polyester and instead increase its thermosetting characteristics. As understood by those familiar with polymers and their properties, a thermoplastic polymer can be melted and resolidified without changing its composition or structure. A thermosetting polymer, however, does not exhibit such change-of-state characteristics and degrades under the application of heat instead of melting. Such thermosetting properties (such as increased strength) are valuable for certain plastics in certain circumstances. Nevertheless, thermoplastic properties are one of the more desirable characteristics of polyester, and reducing those properties is generally considered undesirable under most circumstances.
Because pentaerythritol is a recognized branching agent, it has been typically added in amounts of about 1,000 ppm although some techniques have used much more (e.g., U.S. Pat. No. 3,576,773). Conventionally, adding more pentaerythritol has been considered to raise the thermosetting properties of the polyester to an undesirable extent. In spite of the disclosures in the '773 patent, however, the market still lacks satisfactory low pill polyester staple or staple blends:
"Thus despite substantial effort and research, fully satisfactory polyester fabrics suitable for bedding and the like which are resistant to the formation of pills and capable of withstanding numerous launderings at institutional or industrial temperature and pH levels are still not available."
U.S. Pat. No. 5,591,516; issued Jan. 7, 1997 at Column 2 lines 20-24.
Therefore, the need exists for a polymerization method and resulting polyester for which the strength characteristics can be more appropriately matched to those of natural fibers to thus enhance the blended characteristics of the resulting yarns and fabrics, and yet which can be manufactured without the disadvantages that to date are exhibited by such lower strength polyester materials.